In the field of transistorized current sources, it has been a general practice to employ current mirrors as current sources. Current mirrors have also been used to mirror a varying input signal current. Although such devices have served the purpose, they have not proven entirely satisfactory under all conditions of service because each type of current mirror has a limited operating range for permissible input currents, unless differently sized devices are used for each different application. This problem is overcome by the present invention.
A current mirror receives an input current into one node, typically into a low impedance, and produces an output current at another node. That output current is a direct function (such as a reproduction or linear scaling) of the input current. In some cases, multiple (equal or unequal) currents may be produced for distribution to different output nodes. Increased output resistance and increased effective open circuit voltage can be obtained by a cascode multiple current mirror configuration. An alternative configuration is the Wilson current source, which utilizes negative feedback in lieu of a cascode configuration. Such a configuration has been found to have improved output compliance, which is the voltage range at the output node over which the current source continues to function as a proper current source. However, with the Wilson current mirror, the accuracy with which the output current mirrors the input current is reduced. The cascoded current mirror has better such accuracy, but reduced compliance. Such devices are described in Gray, P. R. and Meyer, R. G. Analysis and Design of Analog Integrated Circuits, Second Edition (John Wiley & Sons, New York 1984) pp. 233-246, 518-519 and 709-718, which is hereby incorporated by reference. Also see Allen, P. E. and Holberg, D. R. CMOS Analog Circuit Design, (Holt, Rinehart and Winston, Inc., New York, 1987) pp. 219-321 (especially pp. 219-239), which is also hereby incorporated by reference.